STEEL FIBRE REINFORCED CONCRETE FOR UNDERGROUND STRUCTURES S. Dutta Dey Dy. Manager – Design & Sector Development, Maccaferri India,
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Dr. Ralf Winterberg Technical Director – Fibre Division, Maccaferri Asia,
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M. Korulla CTO, Maccaferri India
[email protected]
A. D. Gharpure COO & Director, Maccaferri India
[email protected] ABSTRACT This paper deals with the use of fibre reinforced concrete (FRC) in underground construction activities. The correct choice of a steel fibre for structural use is a challenge to engineers and researchers. It is to be remembered that the appropriate choice of fibres strongly depends on the requirements of the application. Wide use in shotcrete and segmental linings worldwide has now and again shown the importance of steel fibres in such applications. Establishing quality assurance of FRC needs to be implemented. For this, proper batching systems and methods need to be enforced in project sites by choosing the right kind of systems based on experience and expertise of leading service providers. 1.0 INTRODUCTION Steel fibres are more and more being used in structural concrete members since they increase the ductility of the concrete matrix. Fibres provide the brittle concrete with a post-cracking strength which can be considered in the structural design. Steel fibres are, in particular, used for providing the post crack load bearing capacity of concrete. Thus fibres are used to improve the overall behaviour at the Ultimate Limit State where they can partially or totally substitute conventional reinforcement. The fibres increase the impact strength and fatigue strength and have a crack-distributing effect so that the cracks formed are smaller. Fibres are used to improve the behaviour in the Serviceability Limit State since they can reduce crack spacing and crack width, thereby improving durability and robustness. Along with steel fibres, special micro polypropylene monofilament fibres are used in concrete for underground applications. On the one hand to improve the plastic shrinkage behaviour due to the ability to hold water on their vast specific surface; on the other hand for passive fire protection, since they markedly reduce or entirely prevent explosive concrete spalling in tunnel fires. The large concrete cover that is often required for fire resistance can in this way be reduced. 2.0 STEEL FIBRES Steel fibres have been prevalently used in various Steel Fibre Reinforced Concrete (SFRC) applications across the globe. They are classified depending on the manufacturing process i.e. cold deformed, cutting, milling or melt extraction or the chemical composition of the base material i.e. low or high carbon content or stainless steel. ASTM A 820-04 [1] and EN 14889-1:2006 [4] categorises steel fibres based on the manufacturing process and their base materials. Steel fibres can have various cross sections from circular to sickle shaped. The surface may be straight, corrugated, grooved or profiled. To ensure proper anchorage into the concrete matrix, the ends of the fibres are usually deformed with double or multiple hooks. 2.1 Steel fibres versus macro synthetic fibres Apart from steel fibres, synthetic fibres are also available commercially. The use of synthetic fibres is application specific. Applications where shrinkage cracks need to be eliminated or reduced or where fire protection is required, micro synthetic fibres show their beneficial performance in concrete. Whereas, for structural applications steel fibres are suited over synthetic fibres, which do not contribute to mechanical load bearing due to their low Young’s modulus. Some market players promote synthetic fibres in lieu of steel fibres projecting low cost as its advantage. These fibres are loosely termed as structural (macro) synthetic fibres. Similar to steel fibres, they bridge “structural” cracks and transfer the tensile stress from one side of the crack to the other, thus exhibiting post cracking
strength in the concrete structure. These synthetic fibres have a low Young’s modulus ranging between 3000 – 5000 MPa, whereas, concrete has a Young’s Modulus of around 30,000 MPa. Hooked Steel Fibers - FF1 - C25/30
4,5
N [MPa]
3,5
Nominal Stress
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2,5
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2,0 1,5 1,0 plain 0.38% 0.51%
0,5 0,0 0,0
0,5
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CTODm [mm]
Fig. 1 –Performance of steel fibres (L/D = 50) (left) and macro synthetic fibres (L/D = 33.5) (right) Due to high Young’s modulus of steel fibres, they start taking load at small crack widths resulting in a strain hardening performance of concrete at displacements below 0.5mm. On the contrary, macro synthetic fibres, due to their low modulus value, establish a full crack pattern before the full strength of the cracked section can be mobilised, which is established in large deformation only. Literature reveals that performance of synthetic fibres over steel fibres are based on toughness testing up to a central deflection as high as 45mm (ASTM C 1550 [2]), whereas most standards including RILEM (2003) [9] restrict the beam test results for deflection of 4mm. Such beam tests provide design values, which are in line with crack width limitations for the Serviceability Limit State (SLS) checks of the codes and standards. Large panel tests as the a.m. accomplish for the flexibility requirements in large deformations as found in the linings of deep mines. A beam test conducted with steel fibre reinforced concrete vs. synthetic fibre reinforced concrete reveals that the structural toughness (area under the load-displacement curve) in case of the latter is very little as compared to Steel Fibre Reinforced Concrete and only starts to develop with large crack width. In most structures, steel fibre reinforced concrete or shotcrete has to carry high stresses with an aim to stabilise ground and restricting movements. It is here that performance at small crack widths is required to fulfil the serviceability requirements on crack width control. In cases where minimal stress carrying capacity is required and the main aim is to keep the concrete together with large deflections and crack widths, synthetic fibres showing nominal toughness value at smaller crack widths but enhanced toughness at higher deflections (toughness calculated for 10, 20 and 40mm central deflection – ASTM C 1550) can be used. For structural use, a minimum mechanical toughness performance of FRC must be guaranteed as stated by the first draft of the New Model code, 2010 [6]. According to UNI 11039 [11], a criterion to define the structural use of Fibre Reinforced Concrete is based on D0 (Ductility Index), Where D0 is computed as: D0 =
feq,(0
f eq,
(0 − 0,6) fI f
= equivalent flexural strength of SFRC, determined in the range of CTODnet (Net Crack Tip Opening Displacement) between 0.0 – 0.6mm f1,f = flexural strength (or limit of proportionality) determined on the basis of the load value corresponding to the CTOD0 value at crack initiation CTODnet = Mean value of the two CTOD values measured on the two opposite faces of the specimen starting from CTOD0 value. – 0.6)
According to UNI, a minimum value of D0 is ≥ 0.5 for the SFRC to behave and to be considered as a structural composite material. In similar lines, the first draft of the New Model code, 2010, criterion is defined by fR1k/flk where fR1k is the characteristic residual strength at CMOD equal to 1.0mm and flk is the characteristic flexural strength at first crack. The draft of the code states that fibre reinforcement can substitute (also partially) conventional reinforcement at ultimate limit state if fR1k/flk > 0.4. If we refer to Figure 1, we can conclude that macro synthetic fibres often do not conform to the above standards of practice and cannot be used in concrete for structural purposes. Hence, it is earnestly questionable to use the term “structural synthetic fibre” for such products. 2.2 Choice of steel fibres
It is often a point of concern to choose the right kind of steel fibre for specific applications. Commercially different kinds of steel fibres viz. drawn wire, slit sheet, mill cut and melt extracted fibres are available. For design and from the structural point of view, the most important parameters to define the fibres’ performance is its aspect ratio, strength and quality of the base material, fibre count (number of fibres per dosage weight) and the type and shape of the anchorage elements. Longer fibres with excessive surface deformations lead to clustering up of fibres a phenomenon called ‘balling’. To avoid this workability problem of fibres, a right balance between performance and workability needs to be attained while designing a fibre. It is found out that a fibre with double end hooks as anchorage elements gives the right optimisation of performance and workability. Considering the strength and quality of the base material; the cold drawn wire fibres have the highest strength of the base materials and boast of high quality levels owing to the manufacturing process. They are in high conformity to the specifications as they can be produced in bulk with very little tolerances according to the referring standards. Due to high quality and strength of the parent material, drawn wire fibres provide higher toughness design values than any other steel fibre. It is to be noted that for high performance concrete a strong fibre with high initial tensile strength needs to be adopted to properly function in post cracking. Fibres with low to moderate strength may yield or rupture due to the tensile loading in the cracks.
Fig 2 - Flexural beam tests (EFNARC) on steel fibre reinforced shotcrete with slit sheet fibre (left) and cold drawn wire fibre (right) Tests on EFNARC (European Federation of Producers and Applicators of Special Products for Structures) [3] thin beams were conducted with cold drawn wire fibres and slit sheet fibres at the same dosage of 37kg/m3. Test results of slit sheet fibres and wire drawn fibres are shown in Figure 2. The slit sheet fibres were 25mm long, 0.8mm wide and a thickness of 0.5mm with a tensile strength of 700MPa whereas the drawn fibres were 30mm long with 0.5mm diameter and strength of 1000MPa. Both fibres have double end hooks as anchorage elements. It is evident from the graphs shown in Figure 2 that slit sheet fibres exhibit strain softening behaviour in post crack whereas a hardening tendency is seen in the tests conducted with cold drawn wire fibres. Thus, indicating superior performance of the wire fibres. Moreover, conformity to harmonised standards like EN 14889 – 1 and the issuance of a certificate of conformity from a notified body gives a quality assurance of the steel fibres used. Generally a CE certification as per EN 14845 – 2 [12] would test the effect of fibres on a reference concrete so as to insure minimum toughness performance: to obtain 1.5 N/mm2 at CMOD (Crack Mouth Opening Displacement) = 0.5mm and 1.0 N/mm2 at CMOD = 3.5mm. 3.0 STEEL FIBRE REINFORCED SHOTCRETE (SFRS) Steel Fibre reinforced shotcrete is gaining tremendous momentum across the globe replacing the conventional mesh shotcrete. It is because there is a lack of a firm and relevant basis for the design of conventional mesh shotcrete. Adhesion of shotcrete to the substrate is a main parameter for primary failure. Designing for that stage means that reinforcement is not needed, or that it is put in as a safety margin for a secondary stage after bond failure. Hence, the next loading stage (or, rather, deformation stage) and relevant design properties, safety margins, etc., need to be identified for the conventional system. Also, mesh shotcrete represents a heavy and laborious technique; requires a more skillful nozzle man to achieve a good quality application, without voids behind the bars and has problems of dust and rebound associated with it. Accurately applied fibre reinforcement provides continuous reinforcement over the whole cross-section of the layer. Thus an even distribution of fibres in the matrix means a positive contribution to the strength, whether the requirement is bending, shear or shock resistance, provided, of course, that the amount and type of fibres are
accurate. Other design aspects, such as shrinkage cracking, are also influenced by fibres. Cracks will be evenly distributed and their widths are small. This is beneficial for several reasons because they restrict early shrinking and fibres may also have a positive effect on the bond to the rock surface. Apart from the usual tests carried out on plain shotcrete viz. compressive strength test, flexural test, bond test to ensure the quality of SFRS, EFNARC [3] has defined an energy absorption test to be carried out on shotcrete panels. These test panels of 600 x 600 x 100mm are cast during the shotcreting process. During testing it is supported at its 4 edges and a centre point load is applied through a contact surface of 100 x 100mm. The toughness requirements are given as specified energy absorption at a certain deflection.
Fig 3 - EFNARC specification on energy absorption requirements 4.0 SEGMENTAL LINING The principle of securing cavities in loose rocks differs from the methods used in hard rock. As loose rock soils are unstable, tunnel driving in these areas must take place with a tunnelling shield that temporarily protects men, machine and materials from the collapsing soil. Geological properties of the excavating soil will determine the type of the tunnel boring machine (TBM) and the process adopted in construction of underground works. The final permanent support – which is generally provided by segmental rings – is constructed under the protection of the shield. Such a ring typically consists of 4 to 8 segments (plus additional keystone), depending on the diameter of the tunnel and prevailing geometrical and technological situation, which form the lining. In the final state, the TBM will carry along the shield, leaving behind the segmental shell that has been installed in the subsoil [13].
Fig.4 – Precast Segment Design Flow Chart The structural design of segmental panels takes into consideration all stages that the panels undergo i.e. from handling, storage, ring assembly, ram thrust and ground loading as shown in Figure 4. Steel fibres can here be used, on the one hand to reduce or substitute the ordinary reinforcement and to manufacture a concrete of WP quality, i.e. for increased density requirements. On the other hand, fibre concrete helps to reduce risks deriving the indicated unexpected load combinations. Spalling resulting from impacts while handling the segments or during their transport can be considerably reduced by the use of fibres. In many cases the ruling loading stage for the design of the segments is ram thrusting, i.e. when the TBM is boring in a forward movement where the hydraulic jacks are pressing against the last installed ring. Imperfections and irregularities, which can occur in practice during TBM operations, need special attention, especially in large bore tunnels [8]. The advantages coming from the use of SFRC in terms of crack control and load-bearing capacity can be beneficially used in the
design. When the use of fibres in this sense is possible, the reduced or replaced reinforcement – compared to the reinforcement required for conventionally reinforced concrete segments – can enable a clearly more costefficient series production, along with a higher quality and robustness of the segments.
Fig.5 – Circular dosing equipments with intermediate weighing and batching unit In segmental lining, to assure proper dispersion and dosage of fibres in concrete, specialised dosage equipments are preferable. Such equipments, integrated in the batching procedure, can feed steel fibres fully automatised into the batching plant to produce fibre reinforced concrete. Through a weighing and vibration system, the doser can feed the conveyor belt or directly into the mixer with a prefixed quantity of fibres, to obtain the mix of SFRC directly inside the batching plant. These equipments are delivered ready to be installed and in particular cases, manufacturers also provide installation support. In large production units of precast segments, the batching cycles are short. However, most of these dosing equipments have a constant dosage rate which may not always match with the batching cycle requirements. Hence a shortfall of fibres into the batching process is a problem encountered with. Officine Maccaferri is the only manufacturer developing and providing specially designed batching system solutions, which have intermediate facilities to dose fibres in order to meet the requirements of high precision and short batching cycles and hence not hindering production time. 5.0 PASSIVE FIRE PROTECTION OF CONCRETE A fire in underground structures not only risks damage to the structure, but also threatens the lives of people. Severe accidents in Europe in recent years have led to a strong demand for the development of fire protection systems in tunnels. Concrete, and especially high performance concrete and shotcrete, are brittle construction materials which can show explosive spalling in response to rapid temperature rises during fires in tunnels.
Fig.6 – Damages in the Gotthard tunnel after the fire in the year 2001 When exposed to fire, the enclosed pore water in concrete is transformed into water vapour. The vaporising pore water in heated concrete leads to an increase of the pore pressure due to the volume increase by the transition of water from liquid to gas state. This vapour pressure causes tensile stresses in concrete that lead to spalling when the tensile strength is exceeded. With increasing strength and quality the density of the concrete increases,
leading consequently to a higher risk of explosive spalling in fire impacts. In conventionally reinforced concrete this can mean the collapse of the structure as the steel reinforcement will become unprotected. The crucial parameter for explosive spalling is the concrete humidity, or the quantity of water in concrete, respectively. Spalling is not expected below a concrete humidity of 2% [7]. The number of pores, the pore size distribution, and their corresponding permeability controls the vapour quantity, which can escape per time unit. Hence the pore structure has a significant influence on the spalling behaviour of concrete. The higher the porosity, the less vapour pressure can be built up and the risk of explosive spalling decreases. An effective reduction of explosive spalling of concrete can be obtained by creating a pore system with sufficient permeability for the vapour pressure, which is building up in fire. Experimental investigations have shown that ultra-fine monofilament polypropylene fibres award beneficial effects to concrete in fire impact [14, 5]. By disintegration in fire impact the fibres build up a connected pore system for the transport of water vapour if the fibres’ fineness and dosage is sufficiently high. The channels thereby created will not be able to develop such a connected system below a certain dosage, but create additional connections within the existing pore structure. Figure 7 shows such a connection channel created by a degraded fibre connecting a pore (smooth surface, to be seen in the right part of the right figure). The remains of the degraded fibre is soot, which is occupying only approx. 5% of the created void.
Fig.7 – Microscope figures showing a partially decomposed fibre (left), and a pore and the transition zone between porous areas, created by a disintegrated fibre (right) The investigations show that concrete spalling rates of up to 300 mm/hour can develop during the course of a fire. A very efficient counter measure is the addition of fine poly-propylene fibres. It was demonstrated that such fibre reinforcement offers the option to create additional porosity in case of fires. Channels are built up following disintegration of the fibres, which increases the permeability of concrete. Furthermore, transition zones between porous areas are created. As a consequence, the concrete becomes more permeable and vapour pressure can expand and escape. A higher cooling effect is reached by increasing the permeability of concrete, due to the energy dissipation of escaping vapour. This leads to lower temperatures inside the concrete.
Fig.8 – Fired surfaces of plain concrete (left) and PP fibre reinforced concrete (3.0 kg/m³, right) after the test The higher the fineness of the fibres, the stronger is this effect. It can be deduced from the present results that the fineness of the monofilament polypropylene fibres is the governing parameter controlling their effectiveness. Figure 8 shows that a dosage of 3.0 kg/m³ of PP fibres with a diameter of 16 microns and a length of 6 mm offer sufficient and reliable safety against explosive spalling in the case of fire. Coarse fibres, or even macro-synthetic PP fibres, do not provide the same mitigating effects [10].
6.0 CASE STUDIES 6.1 First Lining – Bologna–Florence High Speed Tunnel Route The Bologna- Florence route is a fundamental part of the Italian High Speed Railway project. This new route, which was begun in 1997, links the new underground stations of Bologna and Florence and includes 78.5 km of new tracks, of which 73.3 km is within tunnels.
Fig.9 –Circular dosing equipment (right) and Wirand FS4 steel fibres in bags (left) For the preliminary set of the vault and wall Wirand steel fibre FS4 was used in preference to other reinforcement systems, such as welded steel mesh. Wirand reduced the installation time and guaranteed the safety of construction workers, before the final tunnel lining was installed. Quality control of the mechanical and physical characteristics of the steel fibres and concrete during production and the continuous monitoring of the installed fibre reinforced concrete through sampling and testing ensured the final product performance and installation met the project design. 6.2 Segmental Lining – Hobson Bay Sewer and Storm Water Tunnel To replace an old reinforced concrete sewer pipe in Hobson Bay near Auckland in New Zealand a tunnel of three kilometres in length with an internal diameter of 3.7m was proposed. Not only the location and appearance of the old structure, but also its deteriorating state led to the decision of replacing it with a tunnel underneath the bay. The tunnel route extended across Hobson Bay from the pumping station in Orakei to Logan Terrace Parnell and the depth of the tunnel varied from 40m underneath the Hobson Bay upto 95m below the Orakei Ridge. The tunnel lining was erected of segments made of pure steel fibre reinforced concrete.
Fig.10 –SFRC precast segments (left); Circular dosing equipment with tailor-made intermediate batching kit (right) Initial design suggested a TBM driven tunnel lined with precast concrete segments reinforced with conventional welded rebar reinforcement (120 kg per segment). To improve the engineering process and reduce production costs, Wilson Tunnelling Ltd put forward the concept of using steel fibres to reinforce the segments. This approach to totally replace conventional reinforcement with steel fibres was developed further and reviewed by the segment design team from Babendererde Ingenieure GmbH, based in Germany. The design values for steel fibre concrete used in calculations and structural analysis were verified from beam tests carried out at the Institute of Construction Materials Technology at Ruhr-University of Bochum in Germany [13]. Two types of steel fibres, one being Maccaferri Wirand FF3, were considered for the beam tests in which the original concrete mix design, as provided by Wilson Tunnelling, was adopted to obtain most real figures. Three fibre dosages of 30, 40 and 50kg/m³ were considered in the tests.
The test results clearly showed that Steel fibre FF3 provided the best results in terms of workability and performance when compared to the other fibre tested having the same diameter but with a longer fibre length. Wirand FF3, with its shorter length and same wire diameter, has 20% more fibres per unit weight, which improves its effectiveness in crack bridging, thus improving its ductility. This has shown to be significant especially in high strength concrete as used for precast segments.
Fig.11 –Tunnel in progress (left) and Erected segmental lining (right) The Hobson Bay Sewer Tunnel was the first earth pressure balance TBM excavated tunnel in New Zealand. Steel fibres significantly improve the impact strength and the robustness of the segments. This is especially essential for segment handling and installation. In addition, the very high productivity rate and the extremely low damage rate of the segments reinforced with Wirand steel fibres resulted in substantial cost savings of approx. 10% of the total project costs. 7.0 REFERENCES 1. 2. 3. 4. 5.
6. 7. 8.
9. 10.
11. 12. 13.
14.
ASTM A820 / A820M – 06, “Standard Specification for Steel Fibres for Fibre-Reinforced Concrete” ASTM C 1550 – 05, “ Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete ( Usually Centrally Loaded round panel)” EFNARC (European Federation of Producers and Applicators of Special Products for Structures) (1996), “European Specification for Sprayed Concrete”, ISBN 09522483 1 X. EN 14889-1:2006 – “Fibres for concrete – Part 1: Steel fibres - Definitions, specifications and conformity “ Kusterle, W.; Lindlbauer, W. et al., 2004: „Brandbeständigkeit von Faser-, Stahl- und Spannbeton“. Republik Österreich, Bundesministerium für Verkehr, Innovation und Technologie (Hrsg.), Straßenforschung Heft 544 (Österreichische Forschungsgemeinschaft Straße und Verkehr, Wien, 2004). New Model Code 2010 – First Draft – “The 2010 fib Model Code for Concrete Structures” ÖVBB Richtlinie Faserbeton, 2002: Austrian guideline on fibre reinforced concrete (Austrian Association for Concrete and Construction Technology, Vienna, 2002). Plizzari, G.A., Tiberti, G., Winterberg, R., 2008: „Aspekte der Bemessung in faserverstärkten Tübbingen“ / “Design aspects in SFRC Tunnel Segments”, Schildvortrieb mit Tübbingausbau / Mechanised Tunnelling and Segmental Lining, Wissenschaftsstiftung Deutsch-Tschechisches Institut (WSDTI), Eigenverlag der GbR Veröffentlichungen Unterirdisches Bauen, Hamburg (Germany) RILEM TC 162 – TDF, “ Test and design methods for steel fibre reinforced concrete”, TC Membership, , Materials and Structures, Volume 36, October 2003, p 560 – 567 Tatnall, P.C., 2002: Fibre reinforced sprayed concrete: The effect on anti-spalling behaviour during fires. “Modern use of wet mix sprayed concrete for underground support” (Proc. Intern. Symp. on Sprayed Concrete, Davos, 22-26 September 2002) pp. 320-328. UNI 11039: 2003 – “Steel fibre-reinforced concrete—Part 1: definitions, classifications, specifics and conformity; Part II: Test method used to determine the early crack strength and ductility indexes” UNI EN 14845-2:2007 – “Test method for fibres concrete—Part II: Effect on concrete” Winterberg Ralf & Vollmann Götz (2009), “Use of steel fibre reinforced concrete in precast tunnel segment production. Pt. 1”, BFT International Journal 75(2009) Volume 4, p.4-15, ref. ISSN: 03734331 Winterberg, R.; Dietze, R., 2004: “Efficient passive fire protection systems for high performance shotcrete”, 2nd International Conference on Engineering Developments in Shotcrete, Cairns, North Queensland, Australia, October 4-6 (Balkema Publishers, Netherlands, 2004).
